The Storm Prediction
Center maintains a database of all tornadoes that occur within the contiguous
United States. This database includes information such as event location,
time, magnitude, and size. Using this information, the following event
probability values are computed:

1. Any Tornado Threat
For each tornado in a radar region, the total area of the tornado (path
length x width) is computed. These areas are summed for the period of
record, providing the total area affected by tornadoes since 1980 for
each radar area. An annual average area value is computed for each region.
Finally, this value is divided by the total area of the radar coverage region
to determine the annual average probability at any point. The inverse of this value
equates to the return interval of the event at any point.

2. Significant Tornado Threat
This value is computed the same as #1. However, only tornadoes of magnitude
F3 or stronger are considered.

Example
Lets assume that the total area affected by tornadoes in the Wichita, KS radar coverage
area was (661 sq.mi.) during the period 1980-2004. That equates to an average annual coverage
of (26.44 sq.mi./yr). The total area of a (124 mi.) radius radar coverage area is (48305 sq.mi.).

(26.44 sq.mi./yr) / (48305 sq.mi.) = 0.055% (the annual average probability of a tornado at any point in the
ICT radar coverage area).

(48305 sq.mi.) / (26.44 sq.mi./yr) = 1827 years (the return interval of a tornado at any point in the
ICT radar coverage area).